Three interrelated longitudinal studies will continue our investigation of the role of family and peers in adolescent drug use: (1) A longitudinal analysis of the social, interpersonal and psychological determinants of changes in drug use patterns among adolescents while in high school. This will be based on a two-wave panel survey carried out in Fall 1971 and Spring 1972 in which data were obtained from adolescent, parents and best school friend. The basic adolescent sample (N equals 8,206) is a multiphasic random sample representative of public secondary school students in New York State drawn from 18 schools throughout the state. (2) A follow-up study of the drug behaviors of youth in the year following high school, based on a third follow-up carried out in 1972-1973 on 1,645 adolescents from the 1972 senior class in our sample. (3) A study of the formation and dissolution of adolescent friendships in relation to patterns of drug use. Based on 959 stable or changing friendship dyads for whom data are available on adolescents while they are friends, before they choose each other as friends, and after they cease being friends. The extent to which association with drug users leads to drug use and to what extent it follows drug use will be studied. The role of parents in the selection of friends and in the nature of the adolescent's response to peer drug pressures will also be investigated.